(1) Field of the Invention
The present invention relates to a multibeam, digitally-modulated, time division switched communication system especially for use with satellite repeaters, and more particularly to method and apparatus for synchronizing and concurrently switching a plurality of digitally-modulated signals at the repeater.
(2) Description of the Prior Art
Where several ground terminal stations transmit signals through a satellite repeater, conventional communication systems employ either frequency division multiplexing techniques where multiple transponders and a global-coverage antenna are used in the satellite, or time division multiple access/space division multiple access (TDMA/SDMA) techniques where multiple transmit/receive spotbeam antennas are used at the satellite. The latter technique advantageously permits the same frequency band to be used between some or all of the ground stations and the satellite. In prior art TDMA/SDMA systems, several earth stations within a limited geographical zone sequentially access the same spotbeam antenna in a time divided manner. Typically, each such antenna communicates with a different geographical zone on the earth's surface. Additionally, the satellite contains a switching matrix which interconnects antennas into pairs for specified intervals and according to a preestablished sequence so that information may flow from a transmitting earth station in view of one antenna to a receiving earth station in view of another antenna. In this regard see, for instance, U.S. Pat. Nos. 3,711,855 issued to W. G. Schmidt on Jan. 16, 1973, and 3,928,804 issued to W. G. Schmidt et al on Dec. 23, 1975.
In satellite communication systems employing TDMA, the various ground stations must be synchronized to permit the sequentially transmitted pulse bursts to arrive at the satellite at the allocated time periods. Various synchronization techniques have been used. For example, in U.S. Pat. No. 3,530,252 issued to J. G. Puente on Sept. 22, 1970 a synchronizing method is disclosed where a low power accessing pulse is continuously transmitted from each station to the satellite. When the corresponding pulse is returned to the station, its position in the satellite time frame is observed and the phase varied until the access pulse appears at the beginning of the time slot assigned to that station. Alternatively, it is known to transmit a reference synchronizing signal from the satellite which is compared at each station with that station's own signal returning from the satellite. In this regard see, for instance, U.S. Pat. Nos. 3,646,444 issued to W. Bitzer on Feb. 29, 1972, and 3,858,007 issued to H. Ganssmantel on Dec. 31, 1974.
The problem remaining, however, is to provide a satellite repeater in a multibeam, digitally-modulated, time division switched communication system which has increased versatility and reliability while achieving increased switching therethrough with minimal onboard control.